This invention relates to the production of charcoal from biomass.
The term xe2x80x9cbiomassxe2x80x9d includes all sorts of woody and herbaceous plant material, such as wood logs, slabs, chips, and bark; and agricultural residues such as corncobs, corn stover, wheat straw, nutshells, and sugar cane bagasse. Biomass may also include the organic fraction of municipal solid wastes, sewage sludge, manure, or other excrement, and the residues of animal husbandry, such as bones and carcasses. The term xe2x80x9cinertxe2x80x9d in the context of the present invention means that such compound, composition or material does not react with biomass, or its byproducts of pyrolysis, at temperatures and pressures attained within the reaction container in the practice of the present invention.
Charcoal is a carbonaceous solid with a fixed-carbon content of 70 wt % or more. Charcoal is usually manufactured from hardwoods by pyrolysis in large kilns or retorts at temperatures below about 500xc2x0 C. When charcoal is heated (xe2x80x9ccarbonizedxe2x80x9d) in an inert environment to temperatures typically above 80xc2x0 C., it loses most of its remaining volatile matter and becomes a nearly pure carbon (see Table 1) with a fixed-carbon content of 90 wt % or more. As used herein, the term xe2x80x9cbiocarbonxe2x80x9d represents both charcoal and carbonized charcoal. Biocarbons possess many unique properties. Both charcoal and carbonized charcoal contain virtually no sulfur (see Table 1) or mercury. Relative to their fossil fuel cousins, these biocarbons are very low in nitrogen and low in ash. Consequently, many carbonized charcoals are purer forms of carbon than most graphites. Unlike coking coals, pitches, crude resids, and other fossil carbon precursors, biocarbons do not pass through a liquid phase during pyrolysis at low heating rates. Ref. 1 and 2. Consequently, biocarbons are inherently porous. They are also amorphous, as evidenced by very little of a turbostratic structure in their x-ray diffraction spectra. Nevertheless, a packed bed of carbonized charcoal conducts electricity nearly as well as a packed bed of graphite particles.
The charcoal yield ychar is defined as ychar=mchar/mbio, where mchar the dry mass of product charcoal and mbio is the dry mass of the biomass feedstock. Unfortunately, this representation of the efficiency of biomass carbonization is intrinsically vague because the chemical composition of charcoal is not defined. A more meaningful measure of the carbonization efficiency is given by the fixed-carbon yield yfC=Ychar*{% fC/(100-% feed ash)}, where % fC is the percentage fixed-carbon content of the charcoal, and % feed ash is the percentage ash content of the feed. This yield represents the efficiency realized by the pyrolytic conversion of ash-free organic matter in the feestock into a relatively pure, ash-free carbon.
A typical yield of charcoal manufactured from hardwoods in a Missouri kiln operated on a 7 to 12 day cycle is about 28 wt % (on a dry basis). This charcoal has a fixed-carbon content of about 70 wt %; therefore the process offers a fixed-carbon yield of about 20 wt % (xcx9c0.28*0.7). Less efficient processes are widely employed in the developing world. Ref. 3. Such processes are among the principal causes of the deforestation of many tropical countries. Thermochemical equilibrium calculations indicate that a fixed-carbon yield of about 30 wt % should be achieved when equilibrium is reached in a pyrolytic reactor operating at 4xc2x0 C. Ref. 4.
In a prior, commonly assigned patent, it is disclosed that high yields are obtained when pyrolysis is conducted at elevated pressure in a closed reactor vessel wherein the hot vapors are held captive and in contact with the hot solid products of pyrolysis. See U.S. Pat. Nos. 5,435,983 and 5,551,958. Charcoal yields were obtained of 42 to 62 wt % with fixed-carbon contents of 70 wt % or higher on a 1 hour operating cycle. Ref. 4 and 5. Also, various agricultural wastes (e.g., kukui nut shells, macadamia nut shells, and pecan shells) and tropical species (e.g., Eucalyptus, leucaena, and bamboo) offered higher yields of carbon than the hardwoods traditionally employed by industry in the USA and Europe. Moreover, the yields of carbon from oat and rice hulls, and sunflower shells were nearly as high as the yields of carbon from hardwoods. Table 2 displays an estimate of the biocarbon production potential of agricultural residues in the USA based on processing of biomass according to U.S. Pat. No. 5,435,983. Over 200 million tons of fixed carbon can be produced annually from the agricultural residue resource. It is estimated that wood residues generated by the forestry industry could be used to produce about 250 million tons of fixed carbon per year. For comparison, about 990 million tons of coal were produced in the USA in 1999.
Accordingly, an object of the present invention is to provide a rapid, efficient and economical process for converting biomass into charcoal.
It is a further object of the present invention to reduce the required external heat input for converting biomass into charcoal.
It is a further object of the present invention to employ the pressurized hot gas effluent of the reactor to generate power via a steam or gas turbine, or a gas engine.
These and other objects and advantages to the present invention will be readily apparent upon reference to the drawing and the following description.
The present invention provides a low-energy input process for the pyrolytic conversion of biomass into charcoal or carbonized charcoal (collectively referred to as biocarbon) and power, comprising the steps of (i) sealing biomass into a container; (ii) pressurizing the container with air; (iii) heating the biomass to cause it to ignite and burn; (iv) releasing gas from the container during combustion and pyrolysis to maintain the pressure within the reactor below a predetermined value Plimit; (v) delivering additional air into the container, if needed, to achieve a temperature of about 400xc2x0 C. or more throughout the bed of biomass material and while controlling pressure at a predetermined value P1 by release of gas from the container; (vi) further releasing a portion of the gas from the container to lower the pressure of the container to a predetermined value P2 lower than P1. If necessary, the cycle of steps (v)-(vi) may be repeated one or more times at successively lower pressure levels as needed to convert the biomass to biocarbon. After the conversion is complete, the remaining gas is released from the container.